Technical Abstract:
Sclerotium rolfsii attacks a wide range of plants throughout the world. It is most severe in tropical and subtropical areas. Southern Sclerotium blight is of major concern in the eastern, southeastern, and southwestern United States and in Mexico. Although severe losses are reported annually in peanuts and other field and vegetable crops, it is considered of minor importance in alfalfa. It occasionally causes light to moderate losses in alfalfa during extended periods of hot, wet weather in silty and sandy soils heavily infested with S. rolfsii.
Symptoms: Diseased plants appear water-stressed, collapse, and turn light brown or tan. The pathogen causes rotting of leaves, stems, and roots. Dead plants are completely void of green color. If alfalfa is planted where S. rolfsii is established, diseased plants may be scattered throughout the field. During warm, wet periods, profuse white mycelium develop at the base of affected plants and on debris on the soil surface. Small, light brown, spherical sclerotia, resembling mustard seeds, form on the upper surface of the mycelium.
Causal Organism: Southern Sclerotium blight is caused by S. rolfsii. Basidiospores of the teleomorph (Athelia rolfsii) occur rarely in the field or in culture and do not appear to be important in the disease cycle. In culture, profuse white mycelia rapidly cover the agar surface. Sclerotia, about 2 mm in diameter, are produced in about a week on potato-dextrose agar. Asexual spores are not produced.
Disease Cycle and Epidemiology: The fungus overwinters as sclerotia in soil or in crop debris. Mycelia arising from sclerotia may colonize residue before infecting plants. Mycelium from infected plants spreads to adjacent healthy plants. The pathogen is also spread in sclerotia-infested soil or irrigation water. Chance of the disease increases when alfalfa follows a more susceptible infected crop.
Management: Cultivars with resistance to Sclerotinia crown and stem rot show resistance to southern Sclerotium blight. When possible, plant alfalfa in fields free of the pathogen. If alfalfa is to be seeded where the disease occurred the previous year, plow deep to bury the sclerotia.